An organic light emitting device usually comprises two electrodes, and an organic material layer interposed therebetween. The organic light emitting device can further comprise an electron- or hole-injecting layer, or an electron- or hole-transporting layer to improve the performances.
Recently, a stacked organic light emitting device which can enhance the light emitting efficiency per unit area by interposing an organic light emitting layer in the pixel regions of the organic light emitting device has been developed.
FIG. 1 illustrates a cross-section of the conventional stacked organic light emitting device. The device shown in FIG. 1 has a stacked structure in which an anode electrode, a light emitting layer and a cathode electrode are typically sequentially repeated. In the stacked organic light emitting device having such the structure, a predetermined voltage is applied between the uppermost cathode electrode and the lowermost anode electrode. Because of the voltage applied between the two electrodes, a vertical current path is formed depending on the applied voltage. Since light is emitted from several light emitting layers, that is, a first light emitting layer and a second light emitting layer, the luminous efficiency per unit area is enhanced compared to the conventional organic light emitting device having a single emission layer.
Forrest et al. suggested a SOLED (Stacked OLED) using an ITO which is a transparent electrode having a high transmittance for effectively transmitting a visible ray from the inside to the outside of the stacked device, as an intermediate conductive layer which is interposed between the light emitting units and acts as an anode electrode and a cathode electrode. [“Three-Color, Tunable, Organic Light Emitting Devices”, S. R. Forrest et al, Science, vol. 276, 1997, p 2009, “A metal-free, full-color stacked organic light emitting device”, S. R. Forrest et al, Applied Physics Letters, Vol. 74, 1999, 305.].
However, the conductive oxide film such as IZO (indium zinc-oxide) and ITO (indium tin-oxide), as a transparent electrode which is usually used as an anode, has a high work function (usually >4.5 eV). Consequently, when it is used to form a cathode electrode, it is not easy to inject electrons from the cathode to the light emitting layer. From this, there occur some problems that the operational voltage of the organic light emitting device is greatly increased, and that critical characteristics as the device such as light emitting efficiency is deteriorated. Accordingly, it has some limitation on the use of the transparent electrode, which is used as an anode electrode, as a common electrode acting as both of an anode electrode and a cathode electrode.
Meanwhile, U.S. Pat. No. 5,917,280 describes a SOLED, wherein a Mg:Ag alloy layer which is a semi-transparent electrode is used as an intermediate conductive layer which is interposed between the light emitting units, and acts as a common electrode, that is, an electrode acting as both of an anode and a cathode.
Generally, in order to accelerate the hole injection, a material having HOMO energy level such that the anode electrode is modulated to have a Fermi energy level similar to the HOMO (highest occupied molecular orbital) energy level of the hole injecting layer, or having a HOMO energy level similar to the Fermi energy level of the anode electrode is selected as a hole injecting layer. However, since the hole injecting layer should be selected taking into consideration not only the Fermi energy level of the anode electrode, but also the HOMO energy level of the hole transporting layer or the light emitting layer, there is some limitation on selection of the materials for the hole injecting layer. Accordingly, even in the case of using the materials used for a cathode such as a Mg:Ag alloy layer as a common electrode, light emitting characteristics are inevitably limited.
In order to overcome the above-described problems, Forrest et al. has suggested a SOLED having a structure as shown in FIG. 2, in which a Mg:Ag alloy layer and ITO are sequentially stacked for an intermediate conductive layer [“High-efficiency, low-drive-voltage, semitransparent stacked organic light emitting device” S. R. Forrest et al., Applied Physics Letters, vol. 73, 1998, p 2399.]. This SOLED allows color temperature calibration of each unit of the light emitting layers, but color temperature control needs a very complicated electrode structure. Further, this SOLED has a drawback that the preparation process is complicated, for example, due to formation of a Mg:Ag/ITO double layer having limited transparency.
Korean Patent Application Publication No. 2005-29824 discloses a stacked organic light emitting device wherein each of the light emitting units stacked as shown in FIG. 3 is linked in the independent current path. However, in such the stacked organic light emitting device, two light emitting units on the right side and the left side of the intermediate conductive layer (220) are formed in an inverting structure and a non-inverting structure, and accordingly, it is substantially only configured to make the same currents to be applied to a unit device simultaneously, which does not allow color temperature calibration.
Meanwhile, the present inventors filed patent applications on an organic light emitting device, comprising an anode containing a conductive layer and an n-type organic material layer positioned on the conductive layer; a cathode; and a p-type organic material layer, which is positioned between the conductive layer of the anode and the cathode, and is joined with the n-type organic material layer to form an NP junction, wherein an electrical barrier against hole injection and/or hole discharge in the anode/organic material layer interface is lowered by controlling the difference between the LUMO energy level of the n-type organic material layer in the anode and the Fermi energy level of the conductive layer in the anode. Therefrom, the hole injection and/or hole discharge performance is improved, thus giving a high device performance, and an electrode formed of various materials, which favorably make the process for preparing the device easier (see, PCT/KR2005/001381 and Korean Patent Application No. 2005-103664). Particularly, the organic light emitting device according to Korean Patent Application No. 2005-103664 has an advantage that Ca, Ca:Ag, Ca-IZO or Mg:Ag materials can be used for the conductive layer of an anode electrode, and the same materials can be used for the anode electrode and the cathode electrode.